Investigation of element segregation and banded structure in 907A steel based on microbeam X-ray fluorescence spectrometry
907A steel is a self-developed low carbon structural steel for marine products,and it has been widely used in the construction of conventional ships.The banded structure defects are usually formed dur-ing the hot rolling of casting billet,which will result in the uneven composition and structure of casting bil-let,thus severely affecting the subsequent processability and use performance of steels.Therefore,the dis-tribution of banded structure must be characterized in order to ensure the quality of 907A steel.At present,the banded structure in 907A steel is mainly analyzed by metallographic rating.The sample surface should be treated by corrosion and the sample preparation is difficult.Moreover,the segregation of elements can-not be reflected.In experiments,the original position statistic distribution analysis method of banded struc-ture and element segregation in 907A steel based on microbeam X-ray fluorescence spectrometry(μ-XRF)was studied.According to the components and elemental content range in 907A steel,the measuring param-eters of μ-XRF were optimized.The spectral standard sample of low carbon steel alloy with similar compo-sition to the testing samples was employed for the correction of quantitative model of instrument.Two-di-mensional content distribution of four main alloying elements including Ni,Mn,Cr and Cu in 907A steel was obtained.It was found that when the scanning step was 10 μm,the two-dimensional content distribu-tion of Ni,Mn and Cr was in good agreement with the banded structure distribution map obtained by the metallographic method,indicating that the banded structure distribution in steel could be analyzed by the μ-XRF method.The statistical analysis showed that the distribution of Cr was relatively uniform with statis-tical segregation degree of about 5％.The segregation of other three elements was obvious,and the statisti-cal segregation degrees were close to or even higher than 10％.Meanwhile,the distribution of various ele-ments on the banded structure was also investigated.It was found that Mn,Cr and Ni on the dark banded structure showed obvious positive segregation.The segregation degrees of elements on the banded structure were all higher than 1.The positive segregation which caused by the enrichment of Ni was most obvious.
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